(1) Field of the Invention
The present invention relates to a plasma ignition system for an internal combustion engine, and specifically to the plasma ignition system wherein an auxiliary circuit serving as a timer is provided at a plasma ignition energy charging means to keep each of thyristors as a switching circuit element for operatively connecting the plasma ignition energy charging means to a corresponding coil and plasma ignition plug in which a spark discharge has been generated by means of a high surge voltage generating and distributing means turned on during a predetermined interval of time after each thyristor is turned on in response to an ignition timing pulse applied thereat and one of the thyristors and the plasma ignition energy charging means is provided for each engine cylinder, so that a favorable plasma ignition can be made even when the voltage drop from a negatively high voltage toward a zero voltage is slow due to an excessive rise in temperature within a discharge gap of each plasma ignition plug and an unmatched discharge occurring at an irregular ignition timing can be prevented.
(2) Description of the Prior Art
A plasma jet ignition system has been developed as a means for providing a positive ignition and more stable combustion of air-fuel mixture without misfire under various engine operating conditions such as light load condition of unstable combustion with a lean air-fuel mixture.
A conventional plasma ignition system comprises: (a) a low DC voltage supply such as a vehicle battery; (b) a transformer having a primary winding connected to the battery and a secondary winding one terminal of which being connected to the primary winding as a common terminal; (c) a contact breaker incorporated between the common terminal of the transformer and ground which turns on and off repetitively in synchronization with the engine rotation so as to generate a considerably high surge voltage at the secondary winding of the transformer; (d) a mechanical distributor having a rotor which rotates in synchronization with the engine rotation and a plurality of contacts, each located at a fixed interval of distance from other two adjacent contacts and each of which circularly comes in contact with the rotor as the rotor rotates with the engine; (e) a plurality of plasma ignition plugs each mounted within a corresponding combustion chamber of engine cylinder; (f) a trigger pulse generator connected to the common terminal of the transformer which receives serial inductive voltage surge pulses which appear at the common terminal of the transformer and shapes the voltage surge pulses; (g) a thyristor whose gate terminal is connected to the trigger pulse generator and which turns on in response to each of the shaped voltage pulses from the trigger pulse generator; (h) a DC-DC converter which boosts the low DC voltage from the battery to a high DC voltage; (i) a capacitor connected to the DC-DC converter for charging the high DC voltage outputted from the DC-DC converter; (j) a first diode connected to one end of the capacitor which conducts the end of the capacitor to ground when the capacitor charges the high DC voltage from the DC-DC converter and which renders the end of the capacitor float with respect to the ground when the thyristor turns on to ground the other end of the capacitor so as to connect the capacitor to one of the plasma ignition plugs in which the spark discharge has occurred; and (k) a plurality of second diodes each connected between the capacitor and corresponding plasma ignition plug for preventing the current flow due to the spark discharge into the capacitor.
When the repetitive switching operation of the contact breaker causes the interruption of an electric current flowing through the primary winding of the transformer, the secondary winding of the transformer produces an excessively high surge voltage having a peak value of -20 through -30 kilovolts with respect to ground potential. This high-peak voltage is supplied into the distributor so that the respective plasma ignition plugs sequencially receive the high-peak voltage via respective high tension cables having high-frequency resistance characteristics.
At this time, each plasma ignition plug generates a spark discharge at a gap between side and central electrodes thereof so that the side and central electrodes substantially conduct each other.
On the other hand, one ignition pulse shaped by the trigger pulse generator turns on the thyristor at each interval of time, so that the high DC voltage charged within the capacitor is fed into one of the plasma ignition plugs where the spark discharge has already occurred via the thyristor and corresponding second diode. Therefore, the plasma ignition plug generates a high-temperature plasma gas and injects the gas into the corresponding combustion chamber to perform a complete combustion of the air-fuel mixture supplied thereinto.
However, there is a problem in the conventional plasma ignition system that in the case when the drop in the voltage across the two electrodes of each plasma ignition plug from a negatively high voltage toward a zero voltage is considerably slow due to an excessive rise in temperature of the gap between both two electrodes (the excessive rise in temperature described above is chiefly caused by the repetitive plasma ignition operations). Therefore, the thyristor turns on in response to the ignition trigger pulse from the ignition pulse generator with the voltage across the gap between both two electrodes of one of the plasma ignition plugs in which the spark discharge has occurred being kept still at a relatively negative high voltage for a long interval of time. Consequently, the thyristor cannot feed the plasma ignition energy charged within the capacitor into the plasma ignition plug to generate the plasma gas thereat for the interval of time described above so that an electric current that holds the thyristor in the turned-on state does not flow through the thyristor on condition that the voltage (resistance) across the gap described above indicates a low minus voltage substantially equal to the voltage across the capacitor immediately after the thyristor turns on and thereby the thyristor returns immediately to the turned-off state.
Furthermore, there is another problem that, since a single set of thyristor and capacitor is used for all plasma ignition plugs in the conventional plasma ignition system, the terminal voltage across the capacitor when the thyristor is turned on is applied across all plasma ignition plugs so that any of the engine cylinders, e.g., in a suction stroke where the voltage across the two electrodes of the corresponding plasma ignition plug to start the spark discharge is relatively low with respect to the ground potential may introduce an unfavorable discharge, i.e., an unmatched discharge.